With rapid advancement of the fabrication technology of a thin film transistor liquid crystal display (TFT-LCD), the LCD is largely applied in various electronic products such as a Personal Digital Assistant (PDA) device, a notebook computer, a digital camera, a video camera, and a mobile phone due to the fact it has advantages of smaller size, less weight, lower power consumption and low radiation. Moreover, the quality of the LCD is ceaselessly improved and the price thereof is continuously decreased since manufacturers aggressively invest in research & development and employ large-scale fabricating equipment. That promptly broadens the applied fields of the LCD.
Please refer to FIG. 1, the circuit structure of the unit pixel is shown. The unit pixel is switched on or off by a thin film transistor (TFT) 10. The gate and source of the TFT 10 are connected separately to a scan line and a data line, and the drain thereof is connected with a storage capacitor Cst and a pixel electrode. When the TFT 10 is turned on by the scan signals, the data signals from the source can be transferred via the drain to the pixel electrode, and next be applied to the liquid crystal layer 12 so as to produce desired images.
In general, in the process of fabricating the LCDs, some bits and small pieces, such as ion particles, maybe fall and attach on the surfaces of the liquid crystal layer or the alignment films. Therefore, when the two sides of the liquid crystal layer are applied with a direct voltage for a period of time, the ion particles will be attracted by the voltage difference, so as to accumulate on the surfaces of the alignment films disposed on two sides of the liquid crystal layer. And after the direct voltage applied to the liquid crystal layer is removed, the accumulated ion particles on the alignment films still can produce an interior direct voltage remained in the liquid crystal layer for a long time, thereby causing the severe problems of image sticking.
For solving the aforementioned image sticking issues caused by driving the LCDs with the direct voltages, the LCDs at present are driven by alternating voltage sources. However, it is noted that, affected by the parasitic capacitance and the coupling capacitance in the unit pixel, some residual direct voltage still occurs across the two sides of the liquid crystal layer, even though applying the alternating voltage to drive it. Please refer to FIG. 2, the waveform of each terminal of the TFT 10 in the unit pixel is illustrated. When the scan signal Vg on the gate is a high level signal Vgh, the TFT 10 is turned on. Oppositely, when the scan signal Vg is a low level signal Vgl, the TFT 10 will be turned off. Following the operations of turning on and off the TFT 10, the data signals on the drain will appear polarity reversing due to the usage of alternating voltage source.
However, it is noted that, due to the affections of the parasitic capacitor (Cgd) between the gate and the drain, the storage capacitor (Cst), and the capacitor (Clc) of liquid crystal layer, the data signal Vdata transferred to the drain usually has a potential shift of ΔV(Cgd, Cst, Clc). As shown in FIG. 2, no matter what the potential level the signal Vs has, the potential level of the data signal Vdata always has a decrement ΔV, thereby causing a direct voltage effect for the unit pixel. Especially, for the various data signals with different gray levels, the potential shifts thereof are all different too, so the direct voltages effects thereof also differ.
Please refer to FIG. 3, the potential shifts of alternating data voltage signals with different gray levels are illustrated. In the case of the data signals with 256 gray levels, the signal waveforms of level 0, level 63, level 127, level 191, and level 255 are shown. Apparently, except the data signal of level 127, most data signals with other gray levels have asymmetric waveforms due to the potential shift ΔV, thereby causing the direct voltage effects in the liquid crystal layer. For solving this issue, a gamma correction circuit is introduced to produce additional compensated voltages according to different gray levels for adjusting the potential levels of the alternating data signals.
As shown in FIG. 3, compared to the original voltage level Vcdc of common electrode, disposed on another side of the liquid crystal layer opposite to the pixel electrode, the data signals Vdata provided is equal to Vcdc±V(0)+V′(0), wherein V(0) is the alternating data signal of level 0, and V′(0) is the compensated voltage signal of level 0. Thus, by adding the gamma correction voltage V′(0) for adjusting the level of the common electrode, the direct voltage level of level 0 is adjusted to Vcdc+V′(0), so as to avoid of affections of the residual direct voltage. Besides, as aforementioned, each data signal with one specific gray level has one different potential shift ΔV, so the necessary gamma correction voltage signals V′(0), V′(63), V′(191) and V′(255) are all different. And in the case of the data signal of level 127, because it has no potential shift, no additional gamma correction item is applied.
However, it is noted that, nowadays, for promoting the displaying performance of the LCDs, in the display panels the electrodes or alignment films disposed on two sides of the liquid crystal layer usually are designed with-different appearances or formed of different material. The typical design, such as Reflective Liquid Crystal Displays (RLCD), Multi-Domain Vertical Alignment LCDs (MVA-LCD), protrusion-slit type LCDs or hybrid-aligned nematic LCDs (HAN-LCD), etc., all have the asymmetric shaped or different material of electrodes and alignment films. Therefore, for these LCDs, the trapping ratios of ion particles are different on two sides of the liquid crystal layer, thereby causing severe “residual direct voltage” and “image sticking” problems. Under this condition, how to overcome the above issues becomes an important research topic at present.